Injection molding is a technology commonly used for high-volume manufacturing of parts made of meltable material, most commonly of parts made of thermoplastic polymers. During a repetitive injection molding process, a plastic resin, most often in the form of small beads or pellets, is introduced to an injection molding machine that melts the resin beads under heat, pressure, and shear. The now molten resin is forcefully injected into a mold cavity having a particular cavity shape. The injected plastic is held under pressure in the mold cavity, cooled, and then removed as a solidified part having a shape that essentially duplicates the cavity shape of the mold. The mold itself may have a single cavity or multiple cavities. Each cavity may be connected to a flow channel by a gate, which directs the flow of the molten resin into the cavity. Thus, a typical injection molding procedure comprises four basic operations: (1) heating the plastic in the injection molding machine to allow it to flow under pressure; (2) injecting the melted plastic into a mold cavity or cavities defined between two or more mold parts that have been closed; (3) allowing the plastic to cool and harden in the cavity or cavities while under pressure; and (4) opening the mold parts to cause the part to be ejected from the mold.
The molten plastic resin is injected into the mold cavity and the plastic resin is forcibly pushed through the cavity by the injection molding machine until the plastic resin reaches the location in the cavity furthest from the gate. The resulting length and wall thickness of the part is a result of the shape of the mold cavity.
Generally speaking, as a liquid plastic resin is introduced into an injection mold in a conventional injection molding process, the material adjacent to the walls of the cavity immediately begins to “freeze,” or solidify and/or cure. As the material flows through the mold, a boundary layer of material is formed against the sides of the mold. As the mold continues to fill, the boundary layer continues to thicken, eventually closing off the path of material flow and preventing additional material from flowing into the mold. The plastic resin freezing on the walls of the mold is exacerbated when the molds are cooled, a technique used to reduce the cycle time of each part and increase machine throughput.
To overcome the problem of freeze off, the injection pressure of the liquid plastic resin as it is introduced into the mold is increased, typically to 103.421 MPa (15,000 psi), or more. By increasing the pressure, the molding machine can continue to force liquid material into the mold before the flow path has closed off. As the pressure required to mold the component increases, the molding equipment must be strong enough to withstand the additional pressure.
Many conventional injection molding operations use shear-thinning plastic material to improve flow of the plastic material into the mold cavity. As the shear-thinning plastic material is injected into the mold cavity, shear forces generated between the plastic material and the mold cavity walls tend to reduce viscosity of the plastic material, thereby allowing the plastic material to flow more freely and easily into the mold cavity. As a result, it is possible to fill thinwall parts fast enough to avoid the material freezing off before the mold is completely filled.
Reduction in viscosity is directly related to the magnitude of shear forces generated between the plastic material and the feed system, and between the plastic material and the mold cavity wall. Thus, manufacturers of these shear-thinning materials and operators of injection molding systems have been driving injection molding pressures higher in an effort to increase shear, thus reducing viscosity. As stated above, injection molding systems typically inject the plastic material in to the mold cavity at melt pressures of 103.421 MPa (15,000 psi) or more.
The molds used in injection molding machines must be capable of withstanding these high melt pressures. Moreover, the material forming the mold must have a fatigue limit that can withstand the maximum cyclic stress for the total number of cycles a mold is expected to run in its lifetime. As a result, mold manufacturers typically form the mold parts from materials having high hardness, typically greater than 30 Rc, and more typically greater than 50 Rc. These high hardness materials are durable and equipped to withstand the high clamping pressures required to keep mold components pressed against one another during the plastic injection process. These high hardness materials are also better able to resist wear from the repeated contact between molding surfaces and polymer flow.
Recently, injection molding techniques have been developed that use lower injection pressures. These lower pressure techniques allow the mold parts to be made of materials having high average thermal conductivities (e.g., greater than 51.9 W/m ° C. (30 BTU/HR FT ° F.)) to improve cooling times and thus shorten cycle times. However, these high average thermal conductivity materials are generally softer (e.g., having an average Rockwell Hardness of less than 30 Rc) than the high hardness materials used for mold parts in typical high pressure injection molding machines. These mold parts may be used in high productivity injection molding machines (i.e., injection molding machines having one or more of thin walled mold cavities (L/T>100), four or more mold cavities, and guided ejection systems). These mold parts may be made of easily machineable materials, such as materials having a milling machining index of greater than 100%, a drilling machining index of greater than 100%, and/or a wire EDM machining index of greater than 100%, as described in International Patent Application Nos. PCT/US12/38744 and PCT/US12/38846, each of which is hereby incorporated by reference herein.
Because the low pressure mold parts may have physical dimensions that are similar to high pressure mold parts, there is a danger that the low pressure mold parts may accidentally be placed in a high pressure apparatus, or otherwise be subjected to high injection pressures or high clamp tonnages, which destroy or deform the low pressure mold by causing instant failure or fatigue failure over time, thus reducing the useful lifetime of the mold parts.